Field of the Invention
The present disclosure relates to a fixing device and an image forming apparatus having the same.
Description of the Related Art
In recent years, an electrophotographic image forming apparatus which can further improve an image quality of output images and is applicable to various types of papers is required. For example, from a relatively large-sized paper such as an A3-sized paper and the like to a small-sized paper such as an A4R-sized paper and a B5-sized paper, which are commonly used, it is required to output paper of various sizes in the electrophotographic image forming apparatus.
In the electrophotographic image forming apparatus, a film heating fixing system is generally known. The film heating fixing system is a system through which a toner image formed on a paper as a recording material is heated via a fixing film to fix the toner on the paper. A film heating fixing system fixing device forms a fixing nip portion between the fixing film and a pressurizing roller by interposing a heat-resistant film (fixing film) between a fixing heater as a heat generating body and a pressurizing rotating member (pressurizing roller). When the fixing heater is energized, the fixing heater generates heat and the fixing film is heated from a back side. Further, the fixing nip portion is heated. When rotating the pressurizing roller, the fixing film is driven to rotate, which enables to convey a paper entering into the fixing nip portion. In this manner, it is possible to keep the fixing nip portion at a constant temperature while forming an unfixed toner image, heating and conveying the paper entering into the fixing nip portion, and fixing the unfixed toner image as a permanent image.
In the film heating fixing system, it is required to stabilize the fixing nip portion at a predetermined temperature with respect to various types of paper. However, with the above configuration, in a case where the small-sized paper passes through the fixing nip portion, a non-paper passing region is generated at an end portion in a width direction of the fixing nip portion even though it is a heat generating region. It is noted that the width direction means a direction which is orthogonal to a paper conveying direction.
In the non-paper passing region, heat is not taken at the fixing nip portion so that the temperature becomes very high (local temperature rise). If the local temperature rise becomes large, thermal damage is easily given to each member so that it is required to prevent an excessive local temperature rise.
As one method to prevent the local temperature rise, when an end portion temperature becomes at a fixed temperature or higher, paper passing is temporarily stopped until the temperature falls by heat radiation. However, with this method, a downtime is caused by the stop of the paper passing, which causes a reduction in productivity.
Further, when passing a paper whose width in a width direction is wide, due to an influence of the heat radiation from the end portion, a region where the temperature drops is generated at the end portion of a paper passing region. When the region of low temperature is generated, toner fixability is deteriorated and density unevenness of the output image becomes large.
On the contrary, a conventional image forming apparatus according to Japanese Patent Application Publication Laid-open No. 2001-183929 comprises heat generating bodies of different heat generation amount in a width direction of the fixing device and one or more temperature sensors in the width direction. Due to this, in accordance with a temperature difference in the width direction measured by the temperature sensor, the conventional image forming apparatus controls an energization ratio of each heat generating body to maintain productivity while reducing temperature unevenness.
However, in the conventional image forming apparatus, control in accordance with the paper size and a heat generation amount distribution of the heater is not performed. Instead, control is performed only by the temperature. Thereby, in the conventional image forming apparatus, optimum control is not performed, which requires to further reduce the local temperature rise of the fixing film and the temperature unevenness in the width direction of the paper passing region.
For example, using a heater whose heat generation amount orientation is uniform in the width direction, a temperature distribution of the fixing film is shown in FIG. 11 and FIG. 12. FIG. 11 shows the temperature distribution of the fixing film in a case where the A4R-sized paper is continuously passed. FIG. 12 shows the temperature distribution of the fixing film in a case where an SRA3-sized paper is continuously passed. FIG. 11 shows that the temperature of the fixing film locally rises 70° C. with respect to the temperature of the paper passing region. Further, FIG. 12 shows that the temperature of the end portion of the paper passing region is reduced 20° C. as compared to that of a center portion.
Contrary to this, using a heater A and a heater B having the heat generation amount orientation as shown in FIG. 13, the energization ratio of each heater is determined based on the temperature difference between the center portion temperature and the end portion temperature. FIG. 14 shows the temperature distribution in a case where the A4R-sized paper is continuously passed in this case. FIG. 15 shows the temperature distribution in a case where the SRA3-sized paper is continuously passed in this case. It is obvious in FIG. 14 that the local temperature rise with respect to the paper passing region is reduced to 30° C., however, it is obvious that this can further be reduced. Further, in FIG. 15, a temperature drop at the end portion of the paper passing region with respect to the center portion is improved to 10° C., however, it is obvious that this can further be improved.
In view of the above problems, the present disclosure mainly intends to provide an image forming apparatus which reduces the local temperature rise of the fixing film and the temperature unevenness in the width direction of the paper passing region.